Prime Focus Lnb Focal Length

[elgizmops]

Hallo I need some advice please.
My Obitron SX12 manual says that the focal lenght is 50 1/8 inches. Is this measured from the face of the lnb where it bolts to the feedhorn.
I have made of my support legs, which are oversize, and need to cut off the excess

 

[Channel Hopper]

Is it a C band or Ku band feed you are setting up ?

Rule of thumb is to have the focal point just inside the feed, about 1cm for Ku and 2cm for C band, so it's worth making up some form of adjustment at one end of the feed arm to get it right.

Most importantly for Ku band on a mesh dish is the boresight accuracy.

 

[elgizmops]

Thanks for answering.
I am using the dish for TV reception

 

[elgizmops]

what is boresight accuracy, sorry I am a complete novice

 

[RimaNTSS]

2 elgizmops: That would help if you post some pictures of your antenna and LNB.

 

[elgizmops]

Hello,
Attached are photos which I hope will help clarify what I am trying to do.
The piece of wood across the top two legs is only for bracing purposes while I try to adjust the focal length, the lower 2 legs are diconnected to estimate how much need cutting off to achieve the 50 1/8 inch manufacturers focal length setting.
All the legs and the brackets to mount are home made as is the lnb mounting disc.

 

[RimaNTSS]

Not bad at all! That seems to be about 3m mesh dish. At the same time for Ku band (yes, you going to receive Ku and with this LN it will not perform as 3 meter. I do not understand why there is need for such big plate of metal where you have adjusted LNB? I would go for something smaller, like on picture but with 4 holes. Do not be in hurry cutting pieces of 4 nice looking INOX pipes. Maybe for the beginning replace them with traded bars, they sell them in shops. After you find correct focal point of antenna than adjust original pipes. Is that new antenna? Or you got it used from somewhere?
Add: And if you make custom LNB holder (like I showed on the picture) than it is always possible to fine adjust LNB closer-further.

 

[elgizmops]

Hello, its a 3.6 metre dish. About 20 years old but never assembled. When I got it there was no packing list and until i followed the instructions through I didnt realise what was missing. I aslo didnt understand some of the terminology so relied on the diagrams in the instructions.. Yes the legs are 12mm stainless pipe for external use,. I am about a mile from the med and this project has taken over 6 months so far, some of the nuts and bolts have already started to rust.

 

[RimaNTSS]

Like your 4 pipes made of INOX, you better change all nuts&bolts also to INOX ones. Try to calculate focal point by entering data here _http://www.satlex.us/en/fdratio-params.html?diam=360&depth=40
And you definitely need possibility to fine adjust LNB.

 

[elgizmops]

I didnt use a 3 leg setup because the manufacturers instructions showed 4 legs. Unfortunately with parts missing I had to try and work out what I needed. It does mean that there is limited lateral lnb adjustment, probably use plastic washers or have to make something else.

 

[elgizmops]

Yes, I will change all the nuts and bolts to stainless. The instructions state a focal length of 50 1/8 inches. My question was where is this measured from, Channel Hopper suggested 1 cm inside the lnb. I take it that this means 1 cm from the face where the feedhorn attaches. he also mentioned the importance of "boresight accuracy". I dont know what he means.

 

[RimaNTSS]

_http://en.wikipedia.org/wiki/Antenna_boresight That basically mean that your dish, which is designed for C-band, should be adjusted very precisely to be able receive Ku-band signals.

 

[Channel Hopper]

Yes, I will change all the nuts and bolts to stainless. The instructions state a focal length of 50 1/8 inches. My question was where is this measured from, Channel Hopper suggested 1 cm inside the lnb. I take it that this means 1 cm from the face where the feedhorn attaches. he also mentioned the importance of "boresight accuracy". I dont know what he means.

Roughly 1cm inside the feedhorn.

Boresight is both centralising of the feed, ensuring the distance to the reflector circumference is equal in all directions, and the path from the LNB, through the feedhorn and down to the exact centre of the reflector is a straight line. One of those small keyring lasers is especially good at seting the boresight up as they usually fit into the feed and the button is activated at the same time.

 

[PaulR]

If it's a C band dish then you'll get very poor results on Ku band.

C band dishes do not need to be made (and therefore are usually not!) as accurately as Ku band dishes. On top of that the mesh used has a hole size which is larger than the Ko wavelengths and so is not elctrically reflective allowing the Ku signals to literally leak out of the back.

 

[Channel Hopper]

The Orbitron antennas have a 'micromesh' reflector material, which is designed to work at both frequency bands

The claimed figures are

GAIN @ 4.2 GHz: 42.2 dBi
GAIN @ 12.2 GHZ: 49.5 dB

On the other thread I have already stated that the upper Ku band (12.5 - 12.75 GHz) is somewhat iffy on these models though.